A controversial new theory claims that many features of the human face are the result of evolved defensive measures against fist fights.

This is not the first time that fist-fighting has been implicated in the development of our physiologies. Back in 2012, scientists made the claim that fists changed the course of human evolution, arguing that "It is...our most important anatomical weapon, used to threaten, beat and sometimes kill to resolve conflict." The paper earned its fair share of criticism, not only because the evidence was circumstantial, but because of its claim that violence underpinned much of human evolution — a perspective many now consider to be outdated, simplistic, and overly male-oriented (for example, some facial features could be the result of sexual selection). The new theory about human faces, which has been published in Biological Reviews, threatens to do the same.

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The Protective Buttressing Hypothesis

According to David Carrier and Michael Morgan, our distant human ancestors exhibited a remarkable number of features that can only be described as protective buttressing. Indeed, when hominids engage in hand-to-hand combat, the face is typically the primary target. The bones in our face, say the scientists, suffer the highest rates of fracture — but they're also parts of the skull that have exhibited the greatest increase in robusticity during the course of our evolution as hominids.

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Indeed, Carrier and Morgan came to this conclusion after taking a look at the skulls of australopiths. Over time, these hominids developed increasingly stronger brow and nasal ridges, cheek bones, and jaws. More technically, and in the words of the researchers:

Specifically, the trend towards a more orthognathic face; the bunodont form and expansion of the postcanine teeth; the increased robusticity of the orbit; the increased robusticity of the masticatory system, including the mandibular corpus and condyle, zygoma, and anterior pillars of the maxilla; and the enlarged jaw adductor musculature are traits that may represent protective buttressing of the face.

To bolster their case, the researchers also used data from modern humans; they analyzed several studies from hospital emergency wards to see how fist-fighting produces facial injuries.

Prior to this study, anthropologists believed that these particular facial characteristics were an adaptation to a tough diet, one that included nuts, seeds, and grasses. This new theory would seem to be a bit more plausible (the diet hypothesis doesn't explain sexual dimorphism, for example). But like the earlier fist hypothesis, more evidence will need to be presented to bolster such a claim.

The researchers also say this is a male phenomenon — one that's resulted in pronounced differences in facial characteristics between the sexes. These reinforcements, say the researchers, evolved as males fought over females and resources. It also may help to explain why modern humans can accurately assess another man's strength and fighting ability from facial shape and vocal quality.

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Interestingly, the human faces is less robust than those of australopiths. The scientists speculate that there's been a decreased need over time for these defensive measures as our arms and upper body have gotten progressively weaker.